1. Field of the Invention
The present invention relates to an electroconductive resin composition exhibiting a high and stable electroconductivity which contains a small filling amount of a specific carbon black and graphite combination as the electroconductive filler which is used to fill a thermoplastic resin and/or a thermosetting resin.
2. Description of the Related Art
Remarkable developments have and are occurring in the field of electronic machines such as computers, VTR, acoustic instruments, home appliances, word processors, etc. In these electronic instruments, due to the high integration density of electronic circuits and devices thereof, microcurrents are used, and thus a problem arises in that an erroneous actuation, etc., may be caused by electromagnetic waves from an external source.
Plastic moldings are frequently used as casings of these electronic instruments to cope with the demands for a lightweight, low cost, mass-produced article, etc., but these plastic moldings do not have a shielding effect against electromagnetic waves, and thus cannot solve the above problem.
In the prior art, as a method for imparting an electromagnetic wave shielding property to plastic moldings, an electroconductive resin has been proposed comprising an electroconductive filler which is filled and dispersed in a resin. As the electroconductive filler, a metal type filler and a carbon type filler are generally employed.
A metal type filler is suitable for imparting a high electroconductivity, but has drawbacks in that the specific gravity thereof is high, the electroconductivity is reduced the losses occurring due to filler cutting during molding, and the metal type filler causes abrasion of the extrusion screws and molds.
A carbon type filler such as carbon black or graphite will not substantially abrade extrusion screws or molds, but it is not suitable for uses in which a high electroconductivity with a volume resistivity value of 1 .OMEGA..multidot.cm or less is required, such as electromagnetic shielding materials, electrodes, etc. Among electroconductive carbon blacks, a small amount of Ketjen Black EC (produced by Ketjen Black International) can provide a satisfactory electroconductivity, but the mechanical strength of the composition is remarkably worsened and the resultant material is very brittle and easily destroyed. Graphite itself has a good electro-conductivity, but when dispersed in a resin, a high electroconductivity can not be obtained unless a large filling hereof is used, and further, a problem arises in that the resistance value has a great amplitude of 10.sup.1 to 10.sup.-1 .OMEGA..multidot.cm, and a stable electroconductivity cannot be obtained (Japanese Unexamined Patent Publication (Kokai) No. 60-118744). Further, when carbon black and graphite are used in combination, although a high electroconductivity can be obtained from a low filling amount, the mechanical strength of the composition is lowered and the moldability reduced. Therefore, when a highly electroconductive resin composition is obtained by using the carbon black and graphite of the prior art, molding becomes difficult and the resin composition obtained has a lowered mechanical strength, particularly a lowered impact strength, and since carbon black and graphite are more expensive and have a higher specific gravity than the resin, a composition obtained by using these fillers is disadvantageous both in terms of having a high cost and in that it is heavy. To improve the moldability, a method has been proposed in which a propylene-ethylene copolymer with a high binder efficiency is formulated (Japanese Unexamined Patent Publication (Kokai) No. 51-17937), and a method in which a resin-coated carbon black and a resin-coated graphite are employed (Japanese Unexamined Patent Publication (Kokai) No. 61-218648), etc. However, these methods do not provide a reduction of the filling amount of carbon black and graphite, and thus did not provide a solution to the above problems. Accordingly, it is desirable that a carbon black and graphite which is capable of exhibiting a high electroconductivity even at a low filling amount, be produced.